Current limiting fuse unit with casing sealing its terminals



Nov. 10, 1964 T, M. COLE ETAL 3,156,301

CURRENT LIMITING FUSE UNIT WITH CASING SEALING ITS TERMINALS Filed Jan. 7, 1960 2 Sheets-Sheet 1 Fig.1

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c 200 20! l5 24 ggfime l o IMPREGNATION 2 Fl fi 5 L INVENTORS v THOMAS M. COLE I ROBERT E. GOODY ATTORNEY 1964 'r. M. COLE ETAL 3,159,801

CURRENT LIMITING FUSE UNIT WITH CASING SEALING ITS TERMINALS Filed Jan. 7, 1960 2 Sheets-Sheet 2 F5. EnLA.

INVENTORS THOMAS M COL ROBERT B. 60

BY we ATTORNEY United States Patent 3 156 891 CURRENT LIMITING hose UNrr wrrrr casnso SEALING ITS TERMINALS Thomas M. Cole, Harrison, N.Y., and Robert B. Goody,

Tenaiiy, Ni, assignors to Federal Pacific Eieetric Company, a corporation of Delaware Filed fan. 7, 196%, Ser. No. 1,020 7 Ciairns. (Qi. Zirll -Hii) The present invention relates to fuses, particularly to current-limiting fuses. Such fuses are used for circuit protection on power lines that are capable of delivering high currents, such as 100,000 amperes, or more. In the event of a short circuit in the protected branch that is connected to the power line through such fuse, the fuse blows before the instantaneous value of current can rise to anything near the current-supply capacity of the power line, hence the name current-limiting fuses.

An object of the present invention resides in improved performance of current limiting fuses, through novel features of construction.

A more specific object of the invention resides in novel current-limiting fuses capable of unusually quick operation in response to short-circuit conditions, such that the let through current is limited to a minimized fraction of the rated current-interrupting capacity of the fuse. Fun ther features of the novel current-limiting fuse reside in its safety against exploding, and in its consistent and quiet current-interrupting characteristics.

Another object of the invention is to provide currentlimiting fuses of novel effective construction well suited to economical manufacture, and to provide a novel meth- 0d of making such fuses.

As will be seen, the illustrative embodiments of the in vention in its various aspects, which are described in detail below, include a number of cartridge fuses encased and sealed in a plastic body that extends across the edges of the metal end caps and the insulating tube of such cartridge fuses. Portions of the metal end caps that project from the encasing plastic body enter bores in confronting surfaces of massive terminal members of the current-limiting fuse, and the encasing plastic body is in intimate contact with these confronting surfaces. In manufacture, the body of plastic is formed in situ as by cast ing a polymer about the cartridge fuses assembled between the terminal members so as to form seals across the edges of the metal end caps and to be intimately in supporting contact with the tubular wall of each of the cartridge fuses. The novel fuses described in detail below have a wrapping of plastic-impregnated fabric, as of fiber glass, polymerized in situ and bonded to the exterior of the plastic body. The wrapper preferably extends about the lateral surfaces of the massive members, so as to extend across the transverse planes where the terminal members abut the fuse-encasing plastic body, although, as will be seen, the impregnated fabric wrapping can be used to advantage even though it does not extend across the main terminal members of the current-limiting fuse.

The nature of the invention in its various aspects will be better appreciated from the following detailed description of the presently preferred illustrative embodiment thereof and modifications which are shown in. the accompanying drawings forming part of this disclosure. In the drawings:

FIG. 1 is a perspective view of a novel fuse embodying various features of the invention;

FIG. 2 is a longitudinal cross-section, along the line 2--2 in FIG. 3, showing the fuse of FIG. 1 drawn to larger scale and with terminal portions broken away;

FIG. 3 is a transverse cross-section along the line 33 in FIG. 2;

FIG. 1A is a partly exploded perspective view of a modified fuse embodying further novel features; and

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FIG. 2A is a fragmentary cross-section of a modification of the embodiment in FIGS. 1, 2 and 3 viewed as in FIG. 2.

Referring now to FIGS. 1, 2 and 3, a novel current limiting fuse is shown having a pair of massive metal terminals 10 and 12, that have bores 14 for receiving the terminals of plural cartridge fuses. Each bore is only deep enough to receive a portion of a cartridge fuse terminal so that the remaining portion of each cartridgefuse terminal projects outside its respective bore. Each cartridge fuse includes a fuse link 16 in a space within a fibre tube 18, the space about each fuse link being filled with refractory granular insulation 19. This insulation preferably contains a minimum of low metallic oxides which melt below 1000 C. Silica is a typical suitable material, although numerous other materials commonly used for cartridge fuses will be found suitable for present purposes. The fuse link 16 has three portions, including a central portion 16a which is defined by holes 16b. Fuse link 16 is of silver strip, and is of reduced crosssection in the regions of holes 161). It is at these points, where the ends of portion 16a join the end portions of the fuse link beyond holes lidb, that blowing of the fuse commences in response to an overload or a short circuit.

Fuse links in are united to the terminals 2% of the cartridge fuses. Each end portion of a fuse link extends through a slot 29a in a metal wafer 29b. The fuse link is bent across wafer Ziib and is gripped against that wafer by a threaded plug 20c. A firm electrical connection is established at this point from the fuse link to the internally threaded metal portion 2% which has a shoulder or stop portion Ziif against which metal wafer 29b is tightened. Internally threaded metal sleeve 2% is also threaded onto insulating tube 13 which tightly abuts shoulder 26f.

This cartridge-fuse terminal assembly is intended to confine gas within the fuse despite the high pressure that develops during blowing of the fuse. A body of insulation 22 encases the four cartridge fuses illustrated, extending across the exposed portion of insulatmg tube 18 and the portions of metal terminals 20 that project outside bores 14. Body 22 is in intimate engagement with the confronting surfaces of terminals 10 and 12, thereby enhancing the confinement of gas pressure developed during the blowing of the fuse. Body 22 is of suitable plastic, cast or molded in situ so as to be in intimate contact with the surfaces that it engages as shown in FIG. 2. In this condition, body 22 forms a seal across the place where insulating tube 18 enters cartridge-fuse terminal portion 202, and it also forms a seal where terminal portion 2% emerges from bore 14. Phenolics and epoxy thermosetting resins are suitable materials to form body 22, and polyesters are preferred in each case containing the usual fillers, catalysts, etc., for molding or casting.

Further enhancing the seals described and strengthening the current limiting fuse, a wrapping 24 of impregnated fabric is provided, extending not only about plastic body 22 but also across the lateral surfaces of terminals 10 and 12. This impregnated wrapping is polymerized in situ so as to be at least in intimate contact with body 22, and in practice, is bonded thereto so as to form an integral part of that body. This wrapping further enhances the confinement of gas pressure by the seals previously described. The plastic impregnation is preferably the same material as that used in making body 22, polyester-impregnated fabric being preferred about a cast polyester body 22.

A modification of the fuse in FIGS. 1, 2 and 3 is illustrated in FIG. 1A. In that figure the same main terminals 10 and 12 are used as appear in FIGS. 1-3, and the same cartridge fuses are included having terminals 20. Plastic body 22' is shown with its bonded and impregnated wrapping 24. The latter is shown as being flush with the end surfaces of the body 22, and that body in turn is long enough to extend over the entire exposed lateral surface of the insulating tube and a portion of each metal terminal 20 of the individual cartridge fuses. Somewhat different from the fuse of FIGS. 1, 2 and 3, the central unit in FIG. 1A is formed separate from metal terminals and 12 which are thereafter force-fitted onto the metal terminals and into abutment with the end surfaces of body 22'.

In the event that the cartridge fuse assembly 2022'-24 should have served its purpose by blowing, the user need not discard the entire fuse but can remove the metal terminals 10 and I2; and the user can then restore his fuse to usefulness by replacing the center fuse assembly unit.

A further modification is illustrated in FIG. 2A where in the particular form of metal end cap 20 in FIG. 2 is replaced by a different form of metal end cap. The fuse link 16 in FIG. 2A is the same as that in FIG. 2 except that it extends along the axis of the insulating tube 18,

rather than slantwise as illustrated in FIG. 2. Terminal i 20 is in the form of a metal thimble having its edge crimpcd all around the wall of insulating tube 18'; and fuse link 16 is joined to this thimble by a soldered connection which seals the end of the thimble previously formed with a slot to receive the fuse link. The same 7 granular filling of insulation is contained in the cartridge fuse of FIG. 2A as that in FIG. 2, and in embodiment of FIG. 2A the same cast body 22 with its impregnated fabric wrapping 24' is used in FIG. 2A as in FIG. 2, prepared in the same way using the same materials and having the same seals as previously described.

It has been mentioned that insulating tube 18, as well as insulating tube 18', are of fiber. A variety of other materials can be used in place of this material, as is well known in the art. For example, tubes 18 of fabric im pregnated with suitable epoxy resins and polymerized can be used to advantage to increase the strength of the tube. This, in turn, withstands greater gas pressures that may develop. The material used should be chosen for usual insulating properties, with particular attention to its resistance to tracking when exposed to the arcing conditions prevailing when the fuse blows.

Fuses of the foregoing construction have performed highly satisfactorily, limiting the current to only a fraction of the avilable current in an alternating current circuit in short-circuit tests. For example, a series of fuses having a nominal rating of 250 amperes when used on 60- cycle, 250 volt power supply line of 25,000 amp. current capacity and 58,000 amp. capacity R.M.S. limited the current to between 10,500 to 14,500 amps. instantaneous peak values, in times of between 0.132 to 0.146 part of a cycle. Fuses of the same size when used on 60-cycle power lines With still higher current capacities, as 77,000 amperes and 108,000 amperes both at 250 and 600 volts,

had slightly higher let-through peak values of current but reached the limiting value of current in 0.05 to 0.08 part of a cycle. The performance in each case was quiet, and the fuse was mechanically undamaged externally.

The foregoing results seem attributable to a distinctive principle of arc interruption, different from usual fuses which are designed with a limited or controlled leakage passage for pressure relief. Fuses according to the present invention are sealed off so as to insure a maximum rise of pressure; and this is effectively contained through the novel features of construction defined. The contained gas pressure may then be higher than in prior practice, is effective to speed up the quenching of the arc. This limits the total amount of energy developed in the fuse during the arcing period and until arc extinction, and helps explain the improved performance.

A. further explanation for the improved performance relates to the novel construction. The abrupt creation of high pressures at the instant of arc initiation conceivably creates an internal mechanical shock, or impact. This is evidently contained effectively without mechanical damage by virtue of the intimate engagement of the encasing plastic body with the cartridge fuse exteriors. If there were even a limited space between the outside surface of the cartridge fuses and a containing solid body, the fuse walls could rupture initially at the peak of internal pressure and the containing body then could be cracked apart. The novel fuse in which the encasing body is cast about the cartridge fuses precludes any such space.

From the brief times involved in the arcing event, it seems clear that impact forces must be resisted. From this point of view the body of a polymerized plastic is of advantage. It may be selected as properly yielding to the limited degree to import high resistance to mechanical shock. Polyesters, cast and polymerized about the cartridge fuses already assembled to the terminals (FIG. 1) or assembled only to the cartridge fuses with a mold protecting the projecting cartridge fuse terminals (FIG. 1A) are readily formulated to be solid and strong; and still be yielding to a degree; and polyesters when cast in situ have the further property of shrinking during polymerization and thus develop a pre-stressed condition that is ideal to resist internal pressures.

Separation of the current into multiple paths through the parallel fuse links produces multiple arcs when the fuse blows. This promotes arc extinction because multiple arcs in parallel are unstable. More important, is the feature that the energy is subdivided and more effectively contained by the construction described. In the practical examples above, silver links of about one-quarter inch wide and .003 inch, in a tube of about inch inside diameter has been found eminently satisfactory. Where current-limiting fuses of larger ratings are made, large numbers of cartridge fuses are used.

In the embodiments of FIGS. 1 and 2A where the main terminals of the current-limiting fuse are permanently fixed to the cartridge fuses, the method of manufacture differs slightly from that for making the embodiment in FIG. 1A. In the former, the main terminals are first assembled by a force fit to the cartridge fuse terminals (although set screws in the main terminals may be used to insure firm contact). Body 22 is formed by placing the cartridge-fuse and terminal assembly in a mold of the same cross-section as the opposed faces of the main terminals, filling the mold with casting mix in liquid or paste consistency, and causing polymerization through use of heat appropriate to the composition used. Pressure may be applied if required by the plastic used, as in the case of phenolics. The unit that comes from the mold is wrapped with preferably plural convolutions of fabric having an unpolymerized impregnation, which then is subjected to polymerizing heat for the appropriate time. In making the embodiment of FIG. 1A, the main cartridge terminals are omitted, the mold having end walls of the same form as the main terminals to shape the ends of body 22'. Thereafter the wrapping is applied and polymerized as previously described, the wrapping in this case being of lesser axial extent so as to be flush with the ends of body 22.

In a preferred example the wrapping is of glass fibre fabric, impregnated with polyester casting resin, filler and a catalyst, and bonds to the body 22 or 22' so as to become an integral outer layer of that body, particularly when the body is of the same composition. In the casting process, the material shrinks about the cartridge fuses and develops a pre-stressed condition that enhances the pressure-containing effect of the cast body being in intimate contact with the cartridge-fuse walls.

In each of the embodiments above, individual cartridgefuses are completed as separate units which are then incorporated into the current-limiting fuses described. In this way the resistances of the separate elements can be accurately measured and the multiple fuses of accurately known matched resistance can be assembled into a current-limiting-fuse. Matching of the resistances is important for optimum performance of the completed unit.

The cylindrical circulating tube of each individual cartridge fuse enhances the strength of the assembly against the confined gas pressure that must be contained when the fuse blows, and against concussion, as was mentioned previously. Avoidance of mechanical failure is significant because of the danger of fractured fuse parts flying with projectile speed. Mechanical failure may also result in heavy sustained arcing where the fuse fails mechanically, so that the fuse then would fail to clear the circuit.

It will be recognized that the foregoing disclosure represents various proved embodiments of the invention in its various aspects but that obviously variations in detail and varied applications of the novel concepts Will be suggested by this disclosure and therefore the invention should be broadly construed in accordance with the spirit and scope of the invention.

Having thus described our invention, what we claim and desire to secure by Letters Patent is:

1. A current limiting fuse including a plurality of cartridge fuses each having a pair of metal terminal caps at the ends thereof, a cylindrical tube of insulation having the ends thereof tightly received in said caps, a fuse link in each tube interconnecting the terminal caps, and granular insulation filling the space about said fuse link in each cartridge fuse, a pair of massive metal terminal members at the ends of said fuses having plural sockets each receiving only an end portion of a respective terminal cap, and a solid unitary body of shock-resistant insulating polymeric resin filling the space between said fuses from one of said terminal members to the other, and in intimate surface-to-surface contact with the entire outside surface of said tubes of insulation and with the portions of the terminal caps extending outside said sockets, said body thereby forming seals across the edges of said terminal caps and of said sockets.

2. A current-limiting fuse including a plurality of parallel-extending tubular cartridge fuses, each fuse having an insulating cylindrical tube, a pair of confronting metal end caps united to and extending about the ends of the tube, a fuse link extending within each tube and electrically inter-connecting said pair of end caps and granular insulation filling the space within each tube about said fuse link, said current-limiting fuse further including a pair of relatively massive metal terminal members each terminal member having bores tightly receiving only the end portion of a respective one of each pair of said end caps and having good electrical and thermal contact thereto, thereby connecting said fuses in parallel, a portion of each member cap projecting outside said massive terminal member, and a unitary cylindrical body of hard, tough and shock-resistant plastic of substantially greater minimum thickness than the wall thickness of said insulating tube, said body of plastic filling the space between said fuses from one of said terminal members to the other and being in intimate surface contact with each tube and with said projecting portions of said metal caps and forming seals between said tube and said caps.

3. A current-limiting fuse including a plurality of parallel extending tubular cartridge fuses, each fuse having an insulating cylindrical tube, a pair of confronting metal caps tightly fitted about the ends of the tube, a fuse link extending within each tube and electrically interconnecting said pair of end caps and refractory granular insulation filling the space within each tube about said fuse link, said current-limiting fuse further including a pair of relatively massive metal terminal members having multiple bores tightly receiving the end portion of a respective one of each pair of end caps and having good electrical and thermal contact thereto, a portion of each metal cap projecting outside said massive terminal member, and a unitary cylindrical body of hard, tough and shock-resistant plastic of substantially greater minimum thickness than the wall thickness of said insulating tube, said body of plastic filling the space between said fuses from one of said terminal members to the other and being in intimate surface contact with each tube and with said projecting portion of said metal caps and forming seals between said tube and said caps, said body having an encircling plastic-impregnated fabric layer integral therewith.

4. A current-limiting fuse unit, including plural tubular cartridge fuses, each having a tubular insulating wall, closed-end metal terminals and a fuse link interconnecting said terminals, a unitary polyester plastic body cast and internally stressed in tension about said cartridge fuses and in intimate engagement therewith everywhere except for only portions of said cartridge fuses projecting from said body, said body forming seals between said terminals and said tubular wall, said body comprising at least an encifcling layer of impregnated fabric.

5. A current-limiting fuse including a plurality of parallel-extending cartridge fuses, each fuse having an insulating cylindrical tube, a pair of confronting metal end caps united to and closing off the ends of the tube, a fuse link extending within each tube and electrically interconnecting said pair of end caps and granular insulation filling the space Within each tube about said fuse link, said currentlimiting fuse further including a pair of relatively massive metal terminal members electrically and thermally in contact with said confronting end caps, respectively, of all said cartridge fuses and said terminal members thereby connecting said fuses in parallel, and a unitary body of hard, tough and shock-resistant plastic of substantially greater minimum thickness than the wall thickness of said insulating tube filling the space between said cartridge fuses from one of said terminal members to the other and being in intimate contact with the entire exterior of each cartridge fuse except for the portions thereof in contact with said terminal members, said body forming seals between said metal end caps and said insulating tube.

6. A current-limiting fuse including a plurality of parallel extending tubular cartridge fuses, each fuse having an insulating cylindrical tube, a pair of confronting metal caps secured to and closing off the ends of the tube, a fuse link extending within each tube and electrically interconnecting said pair of end caps and refractory granular insulation filling the space within each tube about said fuse link, said current-limiting fuse further including a pair of relatively massive metal terminal members having good electrical and thermal contact to said end caps of said cartridge fuses, respectively, thereby connecting said fuses in parallel, and a unitary body of hard, tough and shock-resistant internally tensioned plastic of the polyester type, filling the space between said fuses from one of said terminal members to the other and being in intimate surface contact with said cartridge fuses, said body forming seals between said metal caps and said insulating tube, said body having at least an encircling layer of polyesterimpregnated glass-fibre fabric integral therewith.

7. A current-limiting fuse, including at least one tubular cartridge fuse having a tubular insulating wall, closed-end cylindrical metal terminals and a fuse link connected to said terminals, a pair of massive metal terminal members at the ends of said cartridge fuse having sockets tightly receiving said metal terminals, respectively, a unitary cast body of polymeric plastic about said cartridge fuse and in intimate engagement therewith throughout except for that portion of each metal terminal projecting from the cast body that is received in a socket, said body being adhered to the confronting surfaces of said metal terminal members, said body forming seals between said cylindrical terminals, said terminal members, and said insulating wall, and a fabric wrapping about said body impregnated with a polymeric plastic bonded to said body and to the adjoining lateral surfaces of said metal termi- UNITED STATES PATENTS Smith Nov. 28, 1922 Hunter et a1 Nov. 21, 1939 Edsall et a1 Aug. 4, 1953 Kozacka Dec. 8, 1953 Swain July 12, 1955 Fisher July 26, 1955 Smith Dec. 20, 1955 Kozacka Apr. 29, 1958 Barnes Feb. 9. 1960 

4. A CURRENT-LIMITING FUSE UNIT, INCLUDING PLURAL TUBULAR CARTRIDGE FUSES, EACH HAVING A TUBULAR INSULATING WALL, CLOSED-END METAL TERMINALS AND A FUSE LINK INTERCONNECTING SAID TERMINALS, A UNITARY POLYESTER PLASTIC BODY CAST AND INTERNALLY STRESSED IN TENSION ABOUT AID CARTIDGE FUSES AND IN INTIMATE ENGAGEMENT THEREWITH EVERWHERE EXCEPT FOR ONLY PORTIONS OF SAID CARTRIDGE FUSES PROJECTING FROM SAID BODY, SAID BODY FORMING SEALS BETWEEN SAID TERMINALS AND SAID TUBULAR WALL, SAID BODY COMPRISING AT LEAST AN ENCIRCLING LAYER OF IMPREGNATED FABRIC. 